Magnetic bubble memories are well known in the art. One mode of operating such memories is called a field-access mode as is also well known. A field access mode bubble memory is characterized by a magnetic field reorienting, typically rotating, in the plane of bubble movement. Bubbles, in response to the cyclical changes in the magnetic field, move along paths defined by magnetically soft elements such as Permalloy or by a repetitive pattern of ion-implanted regions.
Ion-implanted magnetic bubbles memories are disclosed in U.S. Pat. No. 3,792,452 of M. Dixon, R. A. Moline, J. C. North, L. W. Varnerin, Jr., and R. Wolfe, issued Feb. 12, 1974. Shown therein is a familiar magnetic bubble memory organization commonly referred to as major-minor organization. A bubble memory organized in such a manner is characterized by a plurality of closed loop paths termed "minor" loops and at least one "major" path or loop. A bubble generator and a detector are associated with the major path, and data, represented by a bubble pattern, are moved between ends of the minor loops and the major path typically at transfer ports.
Propagation patterns in ion-implanted bubble memories comprise a pattern of bulges and cusps formed typically by a series of unimplanted contiguous discs in an otherwise implanted region. Bubbles adhere to the boundary between implanted and unimplanted regions, and in response to the reorienting in-plane field, move from cusp to cusp along the path of propagation.
The major path can have any one of a variety of geometries. U.S. Pat. No. 4,238,836 of T. M. Burford, filed Mar. 7, 1979, shows a G-shaped path closed into a loop. The major loop is organized in such a manner that bubbles can be transferred out of one end of the minor loops and into the other end. U.S. Pat. No. 4,249,249 of P. I. Bonyhard and T. J. Nelson, filed Dec. 3, 1979, shows the major loop folded into a U-shaped configuration enclosing the minor loops between the legs of the U.
Whatever the configuration of the major loop, the minor loops generally comprise parallel closed paths formed by the above-mentioned contiguous discs. For transfer to occur, the ends of the minor loops must be in proximity to specific portions of the major path. To allow for maximum storage density, the period along the major loop is taken equal to the period within the minor loops and the separation distance between minor loops is typically one period. In other words, in a circuit with an 8.mu. period, the distance between corresponding bulges in adjacent minor loops is 8.mu..
It is desirable to produce bubble memories with as wide operating margins as possible. At least, theoretically, operating margins may extend from bubble "collapse" at high bias fields to bubble "stripout" at low bias fields where the bias field is the familiar field antiparallel to the magnetization of the bubble and operative to maintain the bubble at some nominal operating diameter. In ion-implanted bubble memories, a considerable portion of the low bias field margins above the normal stripout value is lost because of the tendency of the bubbles to stripout along charged walls that extend from one minor loop to an adjacent minor loop for certain directions of the in-plane field. Consequently, the theoretical margins have not been realized or even closely approximated in ion-implanted bubble memories. The problem thus, is to provide an ion-implanted magnetic bubble memory with an increased low bias margin range.
This problem is most acute in the region occupied by the minor loops because, as mentioned before for high storage density, the separation between adjacent loops is small. There, if charged wall enhanced stripout occurs, the end of the strip domain, not propagating along the minor loop to which the bubble belongs, is attracted to the implant boundary forming the propagation path of the next adjacent minor loop. Specifically, during portions of the propagation cycle when the drive field is oriented along one of three axes determined by the crystal symmetry, bubbles tend to "lash out" from bulges of a minor loop to cusps of an adjacent minor loop and vice versa. Once the two ends of an elongated bubble are "anchored" in two different minor loops, control is lost over which loop the bubble eventually will occupy. In this way, information is inadvertently lost or scrambled.
In the interest of higher storage capacity, one endeavors to create memories with increasingly smaller propagation periods and close spacing between loops. However, in ion-implanted memories, stripout between adjacent minor loops is a limitation on minor loop spacing.